Obesity is a major health problem spreading at an epidemic pace throughout the world without any sign of abatement. Development of obesity, which is often associated with insulin resistance and diabetes, results from an excess of energy intake over expenditure. Brown adipose tissues (BAT) have the unique ability to burn excess calories, and facilitate triglyceride clearance and glucose disposal. Previously, we identified Follistatin (Fst) as a direct target of testosterone that regulates skeletal muscle mass and inhibits transforming growth factor- (TGF-) signaling. Based on our preliminary findings, we hypothesize that Fst promotes overall thermogenic program and improves symptoms of obesity and metabolic disorder by regulating overall Mst/TGF-/BMP/Myf5/PRDM16- signaling pathways. We will test our hypothesis with the following Specific Aims- Aim 1: We will demonstrate the essential role of Fst during brown fat differentiation and the regulation of thermogenic program in vitro and in vivo. Aim 2: We will determine the molecular mechanisms by which Fst regulates the overall thermogenic program and Aim 3: We will generate mice expressing Fst under the control of adiponectin or UCP1 regulatory elements, and compare their metabolic parameters and response to high fat chow diet with control littermates. We will determine the effect of endogenous and exogenous Fst on protein and gene expression profiles of key thermogenic markers, and overall cellular bioenergetics using both in vitro and in vivo models under basal and -adrenoceptor agonist (CL316,248) stimulated conditions. Involvement of Myf5/PRDM16, Mst/pSmad2/3/BMP/COX 2, insulin receptor and AMPK/PGC-1? signaling pathway in in vitro models as well as in adiponectin-Fst and UCP1-Fst transgenic mice will be analyzed by Affymetrix gene expression and quantitative western blot analysis. Expression levels of Fst, Myf5 and Smad3 in mouse preadipocytes and MEF cultures will be inhibited by siRNAs and their thermogenic capabilities will be determined. We will generate Fst-transgenic mice using adiponectin (Adipoq) and UCP1-specific promoters, and test the effect of high fat diet on their body composition (Micro CT), energy expenditure (indirect calorimetric analysis), insulin sensitivity and glucose tolerance under both basal and ?-adrenoceptor agonist (CL 316,248) stimulated conditions. Serum levels of Fst, adiponectin and lipid profiles will be analyzed by ELISA. Evaluating the critical role of Fst during its regulation of overall thermogenic process will provide rationale for novel therapeutic drug design for the treatment of obesity and related metabolic diseases.